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If I remember right, this driver is quite picky about signal times and needs 1,8us per signal or it can not safely be assumed to be taken. SO stepper high delay should not be more then 2. More is wasting required comp. time.

With 0 delay you can reach about 40Khz with quadstepping. With delays it gets less. 85mm@200stepspermm => 17000Hz. Double step frequency is important here. With higher delays it must be decreased.

An alternative hardware solution would be RADDS+Due+RAPS128. That would allow 80000Hz without doublestepping. 160Khz with doublestepping.

The RAPS128 need no delays to work. The normal delay we have is long enough.